The present invention generally relates to data communications and, more particularly, to an asymmetric modem communications system and method for achieving high speed data transfers through a telephone network that includes both digital and analog communications mediums.
A telephone network is often used as an interface between a digital modem and an analog modem. Generally, a digital modem is a device that communicates digital data by using digital signals that replicate analog waveforms. An analog modem is a device that communicates digital data by encoding the data on analog waveforms.
FIG. 1 shows a typical telephone network 99 for interconnecting a digital modem 101 and an analog modem 102. The digital modem 101 is usually interconnected with a digital network 113 via digital connections 112a, 112b. For instance, the digital modem 101 may be interconnected to a digital network 113 in the form of a public switch telephone network (PSTN) via a Local Exchange Carrier (LEC) subscriber loop. The digital network 113 may comprise, among other things, a T1 carrier system, a basic rate or primary rate Integrated Services Digital Network (ISDN), a fiber optic cable network, a coaxial cable network, a satellite network, or even a wireless digital communications network. Communications over the digital network 113 are conducted in accordance with a pulse code modulation (PCM) scheme. Channel capacity through these digital facilities is typically between 56 and 64 kilobits per second (kb/s). Coding of the signals is also employed so that compression and a constant signal/distortion performance over a wide dynamic range is achieved for optimal transmission of voice signals. A commonly used coding technique is a nonlinear mu-law coding.
The digital network 113 is in turn interconnected with another LEC subscriber loop that includes a coder/decoder (codec) 106. The codec 106 is interconnected with the digital network 113 via digital connections 114a, 114b. The codec 106 is often situated at a telephone company office or along a street near the analog modem subscriber in an SLC device. The codec 106 provides an interface between the digital network 113 and an analog telephone connection 118, sometimes referred to as a copper loop. For communications in the direction from the digital network 113 to the analog modem 102, the codec 106 includes a mu-to-linear-analog converter 109, which includes digital-to-analog (DAC) conversion functionality. The converter 109 converts nonlinear mu-law levels to a linear analog signal. For communications in the direction from the analog modem 102 to the digital network 113, the codec 106 includes a linear-analog-to-mu converter 107, which includes analog-to-digital (ADC) conversion functionality. The converter 107 converts the linear analog signal to nonlinear mu-law levels.
A hybrid 103 is in communication with the DAC and ADC via respective LPFs 111, 105. The hybrid 103 serves to separate the bidirectional analog signals from the analog telephone connection 118 into unidirectional transmit and receive analog signals sent to and received from the ADC 107 and the DAC 109 respectively.
Furthermore, the analog modem 102 is connected to the analog telephone connection 118 and communicates analog signals therewith. Thus, communications occur between the digital modem 101 and the analog modem 102 by way of the digital network 113 and the codec 106.
Researchers have been attempting to increase the speed at which data can be transferred through the telephone network between the digital and analog modems 101, 102. U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. describes a high speed analog modem 102 that is synchronized to the DAC and ADC clocks of the codec 106. Further, a pulse level modulation scheme is utilized to communicate data along the telephone connection 118. With pulse level modulation, a plurality of voltage levels are communicated along the analog telephone connection 118. This system permits data transfer rates above 40 kb/s.
Although the aforementioned system is meritorious to an extent in terms of increasing data transfer rates, it suffers from various undesirable problems and disadvantages.
A primary disadvantage of the Ayanoglu system involves echo problems. Generally, there is sensitivity to quantized echoes because detection occurs at the codec quantizer, and there is an inability to provide echo cancellation prior to detection. More specifically, echo cancellation at the analog modem 102 is not a problem given its exceptional linearity. However, the echo at the codec is a major problem due to the mu-law coding and limited hybrid quality. On a poor subscriber loop, the receive signal is attenuated. The echo is increased due to the impedance mismatch. In fact, the echo level can exceed the receive signal level. Accordingly, both the analog modem 102 and the digital modem 101 will attempt to utilize all PCM levels. When the digital modem 101 echo results from one of the upper compander levels and the analog modem 102 has transmitted on one of the lower levels, then the echo will control the channel bank encoder step size. In this case, it is difficult to resolve the symbols from the analog modem 102.
Another disadvantage of the Ayanoglu system is that it requires complex timing synchronization with the codec.
Hence, there exists a need in the industry for systems and methods for increasing the speed of data transfers through a telephone network 99, which comprises both a digital and analog communications mediums, between a digital modem 101 and an analog modem 102.
The invention provides for an asymmetric modem communications system and method for achieving high speed data transfers through a telephone network that includes both digital and analog communications mediums. In general, the system includes means for concurrently communicating first and second signals, respectively, in opposite directions along the connection between the communications devices and means for modulating the first and second signals with different modulation techniques. The communications occur in full duplex manner.
In a possible implementation, a digital modem is interfaced to a digital network. The digital network is connected with a coder/decoder (codec). The codec is interfaced with a two-wire analog telephone connection, sometimes referred to as a copper loop. Finally, the telephone connection is interfaced with an analog modem.
Both the digital and analog modems have a transmitter and a receiver. The digital modem has a transmitter that pulse modulates digital data in the sense that it generates and transmits pulse levels and a receiver that receives and demodulates signals in accordance with the standard V.34 communications protocol. Generally, the V.34 protocol employs a form of quadrature amplitude modulation/demodulation. The analog modem has a transmitter that transmits and modulates signals in accordance with the V.34 communications protocol and a receiver that demodulates the pulse levels into digital data.
Communications over the digital network are conducted in accordance with pulse code modulation (PCM). Moreover, communications over the analog connection occur via encoding of digital data on analog waveforms.
With the foregoing configuration, asymmetric data communications are realized. Specifically, the analog modem communicates to the digital modem using the V.34 communications protocol at a data rate of between 33,600 b/s and 2400 b/s, inclusive, while the digital modem communicates to the analog modem at a data rate of between 64,000 b/s and 2400 b/s, inclusive.
Worth noting is that the invention can also be broadly viewed as providing a method for bidirectionally communicating information between first and second communications devices along a connection. The method can be summarized as follows: concurrently communicating first and second signals in opposite directions along the connection between the first and second communications devices, and modulating the first and second signals with different modulation techniques.
The invention has numerous advantages, a few of which are delineated hereafter, as examples.
An advantage of the invention is that data transfers as high as 64 kb/s are achieved through the telephone network from the digital modem to the analog modem, while data is transferred in the reverse direction at up to 33,600 b/s using a conventional V.34 communications protocol.
Another advantage of the invention is that it provides for full duplex communication of signals along the analog telephone connection by using two different modulation techniques, one for each signal that is transferred in opposite directions.
Another advantage of the invention is that because of the two types of modulation that are utilized, echo distortion is minimized. Conventional V.34 modulation utilizes low symbol rates (approximately half) as compared to that described in U.S. Pat. No. 5,394,437 to E. Ayanoglu et al. Thus, each symbol is spread over more than 2 PCM samples so that quantized echo distortion is reduced. Further, V.34 modulation provides detection at the output of a trellis code Viterbi decoder after echo cancellation, receive equalization, precoding, and flexible symbol timing recovery. Finally, V.34 utilizes conventional transmit power levels with modest preemphasis, but no pre-equalization which causes high peak signal levels.
Another advantage of the invention is that it provides for higher speed transactions when a user is interacting with the internet or a computer controlled database.
Another advantage of the invention is that it is simple in design and reliable in operation.
Another advantage of the invention is that it can be implemented with only minor modifications to existing modem designs.
Other objects, features, and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional objects features, and advantages be included herein within the scope of the present invention, as defined by the claims.